Planar Lightwave Circuit Type Variable Optical Attenuator

ABSTRACT

A planar lightwave circuit type variable optical attenuator with a small polarization dependent loss is provided. By setting the waveguide birefringence (absolute value) in first and second optical coupler sections equal to or greater than 3.5×10 −4 , the polarization mode coupling is made equal to or less than −25 dB, and the effect of the polarization dependence caused by the polarization mode coupling at the cross port of the first and second optical couplers is suppressed. In addition to or independently of this, the arm waveguide length can be designed to be equal to an integer multiple of the optical beat length obtained by dividing a used optical wavelength by the waveguide birefringence.

TECHNICAL FIELD

The present invention relates to a planar lightwave circuit typevariable optical attenuator composed of optical waveguides on asubstrate. More particularly, the present invention relates to apolarization independent planar lightwave circuit type variable opticalattenuator that suppresses polarization mode coupling by setting thewaveguide birefringence in component optical couplers at above a certainvalue, or that suppresses polarization dependent loss by setting thelength of arm waveguides at an integer multiple of the birefringenceoptical beat length.

BACKGROUND ART

Recent requirements for greater communication capacity stimulate thedevelopment of optical wavelength division multiplexing communicationsystems (WDM systems) using a plurality of optical wavelengths. Theoptical wavelength division multiplexing communication systems arerequired to equalize the levels of individual wavelength signals fromthe viewpoint of nonlinearity suppression and crosstalk suppression. Atpresent, to achieve the level equalization, planar lightwave circuittype variable optical attenuators are about to be used widely. Since theplanar lightwave circuit type variable optical attenuators can be easilyintegrated such as by arraying, they have an advantage over bulk type,magnetooptics type, or MEMS (Micro Electro Mechanical System) typevariable optical attenuators from the perspective of economic orminiaturization.

A planar lightwave circuit type variable optical attenuator will bedescribed with reference to the accompanying drawings. FIG. 8 is a planview showing a typical conventional planar lightwave circuit typevariable optical attenuator. The planar lightwave circuit type variableoptical attenuator 100 has input waveguides 101 a and 101 b, a firstoptical coupler 102, two arm waveguides 103 and 104, a phase controller105 placed on the arm waveguides, a second optical coupler 106, outputwaveguides 107 a and 107 b, and a thin film heater 108. The referencenumeral 110 designates a stress-releasing groove, which will bedescribed later.

FIG. 9 is an enlarged sectional view taken along the line IX-IX of FIG.8 when a conventional example having no stress-releasing grooves 110 issupposed. As shown in FIG. 9, the planar lightwave circuit type variableoptical attenuator 100 employs a silicon substrate 109 having excellentthermal conductivity as its substrate, and has a structure in which thethin film heater 108 is placed on the surface of the embeddedsilica-based waveguides 103 and 104.

The operational principle of the planar lightwave circuit type variableoptical attenuator 100 will be described briefly. The light entering theinput waveguide 101 a is divided into two parts through the firstoptical coupler 102, and they are fed to the two arm waveguides 103 and104. The light beams traveling through the arm waveguides 103 and 104having the phase controller 105 are combined again through the secondoptical coupler 106. In the course of this, they interfere with eachother so that the light is output from the cross port output waveguide107 b when their phases are in phase, from the through port outputwaveguide 107 a when their phases are out of phase by an amount π witheach other, and from both of the two output waveguides 107 a and 107 bin accordance with their phase difference when the phase difference isbetween zero and π. The phase relationship between the two light beamswhen they enter the second optical coupler 106 is controlled by thephase controller 105 placed at the arm waveguide 104. As the phasecontroller 105, a thermooptic phase shifter is often used which iscomposed of the thin film heater 108 placed on the silica-basedwaveguides 103 and 104. Since the thermooptic effect is a phenomenonthat has no polarization dependence theoretically, it has acharacteristic of having a smaller polarization dependence than theelectrooptic effect or photo-elastic effect.

As described above, since the conventional planar lightwave circuit typevariable optical attenuator utilizing the thermooptic effect canfacilitate integration such as arraying, it has an advantage over thevariable optical attenuator utilizing other technology such as theelectrooptic effect or photo-elastic effect from the standpoint of costand size reduction.

In practice, however, the conventional planar lightwave circuit typevariable optical attenuator utilizing thermooptic effect has a problemof increasing the polarization dependent loss (PDL) when the attenuationof the variable optical attenuator is increased. FIG. 10 illustratesrelationships between the optical attenuation and PDL of the variableoptical attenuator with the cross-sectional construction in FIG. 9. Asillustrated in FIG. 10, a large PDL of nearly 4 dB occurs at the opticalattenuation of 15 dB. The large PDL at the optical attenuation offers aserious problem in the operation of a current optical communicationsystem that does not specifies the polarization state in an opticalfiber. It has been the greatest factor of preventing the planarlightwave circuit type variable optical attenuators from spreading.

Thus, the conventional planar lightwave circuit type variable opticalattenuator has a problem to be solved in that the optical attenuator hasa large polarization dependent loss when the optical attenuation of thevariable optical attenuator is increased.

Non-patent document 1: Y. Inoue et al., “Polarization sensitivity of asilica waveguide thermo-optic phase shifter for planar lightwavecircuits”, IEEE Photon. Technol. Lett., vol. 4, no. 1, pp. 36-38,January 1992.

Non-patent document 2: KIM et al., “Limitation of PMD Compensation Dueto Polarization-Dependent Loss in High-Speed Optical TransmissionLinks”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 1, January 2002.

DISCLOSURE OF THE INVENTION

An object of the present invention to provide a planar lightwave circuittype variable optical attenuator with a small polarization dependentloss by solving the problem of the polarization dependent loss of theplanar lightwave circuit type variable optical attenuator.

To accomplish the foregoing object, according to a first aspect of thepresent invention, there is provided a planar lightwave circuit typevariable optical attenuator having waveguides formed on a substrate, theplanar lightwave circuit type variable optical attenuator comprising: aninput waveguide; a first optical coupler; a second optical coupler; twoarm waveguides connecting the first optical coupler to the secondoptical coupler; and an output waveguide, wherein each of the firstoptical coupler and the second optical coupler is a directional couplerhaving a region in which the two arm waveguides are brought in closeproximity to each other; and a polarization mode coupling in the firstoptical coupler and the second optical coupler is equal to or less than−25 dB.

Here, the absolute value of the waveguide birefringence at opticalcoupler sections constituting the first optical coupler and secondoptical coupler can be made equal to or greater than 3.5×10⁻⁴.

The first optical coupler and second optical coupler can be adirectional coupler constructed by bringing the two arm waveguides inclose proximity to each other.

The length of the arm waveguides can be designed to be equal to aninteger multiple of an optical beat length obtained by dividing a usedoptical wavelength by the waveguide birefringence.

Preferably, at least one of the two arm waveguides can have a phasecontroller; and thereby the planar lightwave circuit type variableoptical attenuator can function as a variable optical attenuator oroptical switch.

Preferably, the substrate can be a silicon substrate, and the waveguidescan be silica-based glass waveguides.

According to the present invention, the foregoing configuration canimplement a planar lightwave circuit type variable optical attenuator,optical switch and optical filter with a small PDL (polarizationdependent loss) at the optical attenuation. As a result, according tothe present invention, the planar lightwave circuit type variableoptical attenuator, optical switch and optical filter that are small insize and good suitable for integration become practical. Thus, thepresent invention contributes to the economization or cost reduction ofcommunication systems such as optical wavelength division multiplexingcommunication systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a structure of a planar lightwave circuittype variable optical attenuator of a first embodiment in accordancewith the present invention;

FIG. 2 is an enlarged sectional view showing an enlarged cross-sectionalconstruction of the planar lightwave circuit type variable opticalattenuator of the first embodiment in accordance with the presentinvention;

FIG. 3A is a process diagram showing a waveguide fabrication process ofthe planar lightwave circuit type variable optical attenuator of thefirst embodiment in accordance with the present invention;

FIG. 3B is a process diagram showing the waveguide fabrication processof the planar lightwave circuit type variable optical attenuator of thefirst embodiment in accordance with the present invention;

FIG. 3C is a process diagram showing the waveguide fabrication processof the planar lightwave circuit type variable optical attenuator of thefirst embodiment in accordance with the present invention;

FIG. 3D is a process diagram showing the waveguide fabrication processof the planar lightwave circuit type variable optical attenuator of thefirst embodiment in accordance with the present invention;

FIG. 3E is a process diagram showing the waveguide fabrication processof the planar lightwave circuit type variable optical attenuator of thefirst embodiment in accordance with the present invention;

FIG. 4 is a characteristic diagram illustrating relationships betweenthe waveguide birefringence and the polarization mode coupling for adirectional coupler cross port;

FIG. 5 is a characteristic diagram illustrating relationships betweenthe optical attenuation and polarization dependent loss (PDL) in theplanar lightwave circuit type variable optical attenuator of the firstembodiment in accordance with the present invention;

FIG. 6 is a characteristic diagram illustrating relationships betweenthe optical attenuation and polarization dependent loss (PDL) in thevariable optical attenuator of a second embodiment in accordance withthe present invention;

FIG. 7 is a characteristic diagram illustrating relationships betweenthe optical attenuation and polarization dependent loss (PDL) in theplanar lightwave circuit type variable optical attenuator of a thirdembodiment in accordance with the present invention;

FIG. 8 is a plan view showing a planar lightwave circuit type variableoptical attenuator based on conventional technology;

FIG. 9 is an enlarged sectional view showing an enlarged cross-sectionalconstruction of the planar lightwave circuit type variable opticalattenuator based on the conventional technology;

FIG. 10 is a characteristic diagram illustrating relationships betweenthe optical attenuation and polarization dependent loss (PDL) in theplanar lightwave circuit type variable optical attenuator of theconventional technology;

FIG. 11 is an enlarged sectional view showing an enlargedcross-sectional construction of the planar lightwave circuit typevariable optical attenuator with stress-releasing grooves based on theconventional technology; and

FIG. 12 is a characteristic diagram illustrating relationships betweenthe optical attenuation and polarization dependent loss (PDL) in thevariable optical attenuator with the stress-releasing grooves of theconventional technology.

BEST MODE FOR CARRYING OUT THE INVENTION

(PDL Generation Model and Necessary Conditions for Suppressing PDL)

Before describing concrete embodiments in accordance with the presentinvention, a researched conclusion will be described of the polarizationdependent loss factor of the planar lightwave circuit type variableoptical attenuator.

It is described before in connection with the conventional technologythat the thermooptic effect in the silica-based glass is basically aphenomenon without the polarization dependent loss. Here, the reason whythe planar lightwave circuit type variable optical attenuator has apolarization dependent loss will be described with reference to FIG. 8and FIG. 9 as follows. The following two factors are conceivable asprincipal factors of the polarization dependent loss. One of them is thepolarization dependent loss of the thermooptic phase shifter 105, andthe other of them is a polarization mode coupling of the opticalcouplers 102 and 106.

First, the non-patent document 1 presents a report on the polarizationdependent loss of the former thermooptic phase shifter 105. Thefollowing is a brief description of the contents of the report. Thesilica-based waveguides 103 and 104 that are made local heat up by thethin film heater 108 are about to expand. In this case, although theycan expand in a direction perpendicular to the substrate 109 (in theupward direction of FIG. 9), they cannot expand in a direction parallelto the substrate 109 (in a lateral direction of FIG. 9) because they aresurrounded with a silica-based glass (cladding) 111 that is not heated.As a result, strong compressive stress occurs in a direction parallel tothe surface of the substrate 109. The compressive stress increases therefractive indices of the waveguides (cores) 103 and 104 because of thephoto-elastic effect caused by the compressive stress. Accordingly, asfor the waveguides 103 and 104 beneath the thin film heater 108, theirrefractive indices increase because of the photo-elastic effect due tothe local thermal expansion of the glass as well as the thermoopticeffect involved in the temperature rise. Thus, although the thermoopticeffect itself has no polarization dependent loss, since the stresscaused by the thermal expansion has anisotropy, the refractive indexchanges due to the photo-elastic effect depend on polarizations.

The polarization dependence of the thermooptic phase shifter because ofthe photo-elastic effect can be suppressed to some extent by formingstress-releasing grooves 110 on both sides of the thermooptic phaseshifter 105 (and thin film heater 108) as shown in FIG. 11. FIG. 12illustrates relationships between the optical attenuation and PDL of thevariable optical attenuator having the stress-releasing grooves 110 asshown in FIG. 11. The variable optical attenuator with thecross-sectional construction of FIG. 9 has a PDL of 3.8 dB at 15 dBattenuation (see FIG. 10). In contrast with this, the variable opticalattenuator with the stress-releasing grooves of FIG. 11 can reduce thePDL to 1.7 dB, less than half the value of 3.8 dB. However, the PDL of1.7 dB at the 15 dB attenuation is not sufficient value for practice useof the current optical communication system, and further suppression ofthe PDL is required. The present invention aims at achieving a PDL of0.5 dB or less at the 15 dB attenuation, which is required for thepractice use of the current optical communication system (non-patentdocument 2).

The stress-releasing grooves 110 of FIG. 11, which are allocated at bothsides of the thin film heater 108, have a function of heat-insulatinggrooves for preventing the heat produced by the thin film heater 108from heating regions other than the waveguides. Accordingly, they areeffective for reducing power consumption of the thermooptic phaseshifter as well.

Next, the polarization dependent loss due to the polarization modecoupling of the optical couplers will be described. Here, we assume asthe optical couplers the directional couplers 102 and 106 constructed byplacing the two waveguides in close proximity to each other as shown inFIG. 8. Generally, as for the waveguides on a plane substrate, thecoupling between polarization modes does not occur unless thedisturbance is present. However, since the cores are placed in closeproximity to each other at the directional couplers, the cores undergoforce in the direction that brings them closeness at burying the coresinto the over-cladding layer. The following is a more specificdescription. When forming the over cladding using a flame hydrolysisdeposition method, during the process of thermal treatment forincreasing the transparency carried out after the deposition of fineglass particles on and around the cores, the fine glass particles coverthe cores while being melted and compacted. However, since the fineglass particles are not supplied sufficiently to the region sandwichedbetween the two cores, the glass in the region becomes rough, and thetwo cores are pressed inward from both the outsides. Since the pressureinclines the principal axes of the waveguides, coupling between thepolarization modes occurs. Thus, some part of the cross port lightcoupled through the directional coupler brings about the polarizationmode coupling. On the other hand, as the two cores separate from eachother, the principal axes return to their original positions, both topand bottom, left and right. Accordingly, the through port light does notcause the polarization mode coupling. Such a phenomenon not only occursin the directional couplers, but also occurs without exception in thecase where the two arm waveguides are placed in close proximity to eachother. Thus, the polarization mode coupling occurs even in a multimodeinterference coupler or asymmetric x-type demultiplexer because the twoarm waveguides are brought into close proximity to each other at theinput/output terminals. Next, the light propagation through the opticalcoupler under the polarization mode coupling condition will be analyzedwith reference to FIG. 8. The light propagating from the first inputwaveguide (input port) 101 a to the first output waveguide (output port)107 a via the first arm waveguide 103 is expressed by the followingexpression (1); the light propagating from the first input waveguide 101a to the first output waveguide 107 a via the second arm waveguide 104is expressed by the following expression (2); the light propagating fromthe first input waveguide 101 a to the second output waveguide 107 b viathe first arm waveguide 103 is expressed by the following expression(3); and the light propagating from the first input waveguide 101 a tothe second output waveguide 107 b via the second arm waveguide 104 isexpressed by the following expression (4).

Here, the first rows of the matrices of the following expressionsrepresent a TE component, and the second rows represent a TM component.In addition, I_(TE (TM)) designates the TE (TM) component of the inputlight; κ designates the coupling efficiency of the optical coupler; αdesignates a gradient of the principal axis in the optical coupler; andθ_(1 (2) TE (TM)) designates a phase change of the TE (TM) component inthe first (second) arm waveguide 103 (104). The cross port polarizationmode coupling in the optical coupler is represented by sin 2α.$\begin{matrix}{{\begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}\sqrt{1 - \kappa^{2}}\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{j{{\cdot \sin}\quad\alpha}} & {\cos\quad\alpha}\end{pmatrix}{\begin{pmatrix}{\mathbb{e}}^{j\quad\theta\quad 1{TE}} & 0 \\0 & {\mathbb{e}}^{j\quad\theta\quad 1{TM}}\end{pmatrix} \cdot \begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}\sqrt{1 - \kappa^{2}}\begin{pmatrix}{\cos\quad\alpha} & {j{\sin\quad\alpha}} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}\begin{pmatrix}I_{TE} \\I_{TM}\end{pmatrix}} = {\left( {1 - \kappa^{2}} \right)\begin{pmatrix}{I_{TE}{\mathbb{e}}^{j\quad\theta\quad 1{TE}}} \\{I_{TM}{\mathbb{e}}^{j\quad\theta\quad 1{TM}}}\end{pmatrix}}} & (1)\end{matrix}$ $\begin{matrix}{{\begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}j\quad{\kappa\begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}{\begin{pmatrix}{\mathbb{e}}^{j\quad\theta\quad 2{TE}} & 0 \\0 & {\mathbb{e}}^{j\quad\theta\quad 2{TM}}\end{pmatrix} \cdot \begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}j\quad{\kappa\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}\begin{pmatrix}I_{TE} \\I_{TM}\end{pmatrix}} = {- {\kappa^{2}\begin{pmatrix}{{I_{TE}\left( {{{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\cos^{2}2\quad\alpha} + {{\mathbb{e}}^{j\quad\theta\quad 2{TM}}\sin^{2}2\quad\alpha}} \right)} +} \\{j\quad{I_{TM}\left( {{\mathbb{e}}^{j\quad\theta\quad 2{TE}} - {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\sin\quad 2\quad\alpha\quad\cos\quad 2\alpha} \\{{j\quad{I_{TE}\left( {{- {\mathbb{e}}^{j\quad\theta\quad 2{TE}}} + {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\sin\quad 2\quad\alpha\quad\cos\quad 2\alpha} +} \\{I_{TM}\left( {{{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\sin^{2}\quad 2\quad\alpha} + {{\mathbb{e}}^{j\quad\theta\quad 2{TM}}\quad\cos^{2}\quad 2\alpha}} \right)}\end{pmatrix}}}} & (2)\end{matrix}$ $\begin{matrix}{{\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}j\quad{\kappa\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}{\begin{pmatrix}{\mathbb{e}}^{j\quad\theta\quad 1{TE}} & 0 \\0 & {\mathbb{e}}^{j\quad\theta\quad 1{TM}}\end{pmatrix} \cdot \begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}\sqrt{1 - \kappa^{2}}\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}\begin{pmatrix}I_{TE} \\I_{TM}\end{pmatrix}} = {j\quad\kappa\sqrt{1 - \kappa^{2}}{\begin{pmatrix}{{I_{TE}{\mathbb{e}}^{j\quad\theta\quad 1{TE}}\cos\quad 2\quad\alpha} + {j\quad I_{TM}{\mathbb{e}}^{j\quad\theta\quad 1{TM}}\sin\quad 2\quad\alpha}} \\{{j\quad I_{TE}{\mathbb{e}}^{j\quad\theta\quad 1{TE}}\sin\quad 2\quad\alpha} + {I_{TM}{\mathbb{e}}^{j\quad\theta\quad 1\quad{TM}}\cos\quad 2\quad\alpha}}\end{pmatrix}.}}} & (3)\end{matrix}$Case of I₁→arm₂→O₂: $\begin{matrix}{{\begin{pmatrix}{\cos\quad\alpha} & {{{- j} \cdot \sin}\quad\alpha} \\{{{- j} \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}\sqrt{1 - \kappa^{2}}\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}{\begin{pmatrix}{\mathbb{e}}^{j\quad\theta\quad 2{TE}} & 0 \\0 & {\mathbb{e}}^{j\quad\theta\quad 2{TM}}\end{pmatrix} \cdot \begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}j\quad{\kappa\begin{pmatrix}{\cos\quad\alpha} & {{j \cdot \sin}\quad\alpha} \\{{j \cdot \sin}\quad\alpha} & {\cos\quad\alpha}\end{pmatrix}}\begin{pmatrix}I_{TE} \\I_{TM}\end{pmatrix}} = {j\quad k\sqrt{1 - k^{2}}\begin{pmatrix}{{I_{TE}{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\cos\quad 2\quad\alpha} + {j\quad I_{TM}{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\sin\quad 2\quad\alpha}} \\{{j\quad I_{TE}{\mathbb{e}}^{j\quad\theta\quad 2{TM}}\sin\quad 2\quad\alpha} + {I_{TM}{\mathbb{e}}^{j\quad\theta\quad 2\quad{TM}}\cos\quad 2\quad\alpha}}\end{pmatrix}}} & (4)\end{matrix}$

The t hrough port output from the first input waveguide 101 a to thefirst output waveguide 107 a is given by the following expression (5),which is the sum of the foregoing expression (1) and the foregoingexpression (2). $\begin{matrix}\begin{pmatrix}{{I_{TE}\left\{ {{\left( {1 - \kappa^{2}} \right){\mathbb{e}}^{j\quad\theta\quad 1{TE}}} - {\kappa^{2}\left( {{{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\quad\cos^{2}2\quad\alpha} + {{\mathbb{e}}^{j\quad\theta\quad 2{TM}}\sin^{2}2\quad\alpha}} \right)}} \right\}} -} \\{{j\quad I_{TM}{\kappa^{2}\left( {{\mathbb{e}}^{j\quad\theta\quad 2{TE}} - {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\sin^{2}\quad 2\quad\alpha\quad\cos\quad 2\quad\alpha} -} \\{{j\quad I_{TE}{\kappa^{2}\left( {{- {\mathbb{e}}^{j\quad\theta\quad 2{TE}}} + {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\sin\quad 2\quad\alpha\quad\cos\quad 2\quad\alpha} + I_{TM}} \\\left\{ {{\left( {1 - \kappa^{2}} \right){\mathbb{e}}^{j\quad\theta\quad 1{TM}}} - {\kappa^{2}\left( {{{\mathbb{e}}^{j\quad\theta\quad 2{TE}}\sin^{2}\quad 2\quad\alpha} + {{\mathbb{e}}^{j\quad\theta\quad 2{TM}}\cos^{2}\quad 2\quad\alpha}} \right)}} \right\}\end{pmatrix} & (5)\end{matrix}$

To improve the outlook in the foregoing expression (5), the followingexpression (6) is assumed as the coupling ratio of the optical coupler.$\begin{matrix}{\kappa^{2} = \frac{1}{2}} & (6)\end{matrix}$

Since the through port output undergoes the maximum attenuation when thelengths of the two arm waveguides are equal, the maximum attenuationcondition is given by the following expression (7).e^(jθ1TE)=e^(jθ2TE), e^(jθ1TM)=e^(jθ2TM)   (7)

By substituting the foregoing expressions (6) and (7) into the foregoingexpression (5), the condition is obtained for always keeping theforegoing expression (5)=0 independently of the polarization state. Itgives the polarization independent condition. Here, the term“independent of the polarization state”means that it depends neither onthe intensity ratio between I_(TE) and I_(TM) or on the phasedifference. $\begin{matrix}{{\frac{\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TE}} - {\mathbb{e}}^{j\quad\theta\quad 1{TM}}} \right)\sin\quad 2\alpha}{2}\begin{pmatrix}{{I_{TE}\quad\sin\quad 2\quad\alpha} - {j\quad I_{TM}\quad\cos\quad 2\quad\alpha}} \\{{j\quad I_{TE}\quad\cos\quad 2\quad\alpha} - {I_{TM}\quad\sin\quad 2\quad\alpha}}\end{pmatrix}} = \begin{pmatrix}0 \\0\end{pmatrix}} & (8)\end{matrix}$

As the polarization independent condition, the following expression (9)is derived from the foregoing expression (8), where m is an integer.sin 2α=0

orθ_(1TE)−θ_(1TM)=2 mπ  (9)

Here, in the first arm waveguide 103, θ_(1TE), θ_(1TM) and B are definedby the following expressions (10), (11) and (12) where L is the lengthof the arm waveguide, λ is the wavelength employed, n_(1 TM) is theeffective index of the TM light, n_(1 TE) is the effective index of theTE light and B is the waveguide birefringence. $\begin{matrix}{\theta_{1{TE}} = {2\quad\pi\frac{n_{1{TE}}L}{\lambda}}} & (10) \\{\theta_{1{TM}} = {2\pi\frac{n_{1{TM}}L}{\lambda}}} & (11)\end{matrix}$  B=n _(1TM) −n _(1TE)   (12)

The following expression (13) is obtained by substituting theseexpressions (10) and (11) into the lower expression (called “secondexpression” from now on) of the foregoing expression (9), and byapplying the expression (12). $\begin{matrix}{L = {{- m}\frac{\lambda}{B}}} & (13)\end{matrix}$

Thus, it is found from the foregoing expressions (9) and (13) that thepolarization dependent loss of the through port output can be eliminatedwhen the polarization mode coupling (sin 2α) becomes zero in the opticalcoupler, or when the length (L) of the arm waveguides is an integermultiple (m) of the optical beat length obtained by dividing the opticalwavelength employed(λ) by the waveguide birefringence (B).

Likewise, the cross port output from the first input waveguide 101 a tothe second output waveguide 107 b is given by the following expression(14), which is the sum of the foregoing expression (3) and the foregoingexpression (4). $\begin{matrix}{j\quad\kappa\sqrt{1 - \kappa^{2}}\begin{pmatrix}{{{I_{TE}\left( {{\mathbb{e}}^{j\quad\theta\quad 1\quad{TE}} + {\mathbb{e}}^{j\quad\theta\quad 2{TE}}} \right)}\cos\quad 2\quad\alpha} +} \\{j\quad{I_{TM}\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TM}} + {\mathbb{e}}^{j\quad\theta\quad 2{TE}}} \right)}\sin\quad 2\quad\alpha} \\{{j\quad{I_{TE}\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TE}} + {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\sin\quad 2\quad\alpha} +} \\{{I_{TM}\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TM}} + {\mathbb{e}}^{j\quad\theta\quad 2{TM}}} \right)}\cos\quad 2\quad\alpha}\end{pmatrix}} & (14)\end{matrix}$

The cross port output undergoes the maximum attenuation when thedifference between the lengths of the two arm waveguides is half theoptical wavelength employed. The condition is represented by thefollowing expression (15).e ^(jθ1TE) +e ^(jθ2TE) =e ^(jθ1TM) +e ^(jθ2TM)=0   (15)

By substituting the foregoing expression (15) into the foregoingexpression (14), the condition is obtained for always keeping theforegoing expression (14)=0 independently of the polarization state. Itgives the polarization independent condition. $\begin{matrix}{{{- \kappa}\sqrt{1 - \kappa^{2}}\sin\quad 2\quad{\alpha\begin{pmatrix}{I_{TM}\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TM}} - {\mathbb{e}}^{j\quad\theta\quad 1{TE}}} \right)} \\{I_{TE}\left( {{\mathbb{e}}^{j\quad\theta\quad 1{TE}} - {\mathbb{e}}^{j\quad\theta\quad 1\quad{TM}}} \right)}\end{pmatrix}}} = \begin{pmatrix}0 \\0\end{pmatrix}} & (16)\end{matrix}$

As the polarization independent condition, the following expression (17)is derived from the foregoing expression (16).sin 2α=0

orθ_(1TE)−θ_(1TM)=2 mπ  (17)

The foregoing expression (17) equals the foregoing expression (9).Accordingly, the foregoing expression (13) is obtained from theforegoing expression (17). Thus, the polarization dependent loss of thethrough port output and that of the cross port output can be eliminatedwhen the polarization mode coupling (sin 2α) becomes zero in the opticalcoupler, or when the length (L) of the arm waveguides is an integermultiple (m) of the optical beat length obtained by dividing the opticalwavelength employed (λ) by the waveguide birefringence (B).

From the foregoing consideration, the conditions are obtained which arenecessary for suppressing the polarization dependent loss of the planarlightwave circuit type variable optical attenuator and optical switch.

The embodiments in accordance with the present invention will now bedescribed in detail with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows a structure of a planar lightwave circuit type variableoptical attenuator of the first embodiment in accordance with thepresent invention. The planar lightwave circuit type variable opticalattenuator 100 has an input waveguide 101 a, a first optical coupler102, two arm waveguides 103 and 104, a phase controller 105 placed onthe arm waveguides, a second optical coupler 106, an output waveguide107 b, thin film heaters 108, and stress-releasing grooves 110. Theembodiment uses the waveguide 107 b, which is located at the cross portwith respect to the input waveguide 101 a, as the output waveguide. Thereason for using the cross port output is that when the directionalcouplers used as the first and second optical couplers 102 and 106 havethe same design, their coupling ratios are nearly equal, as a result ofwhich a high variable optical attenuation is achieved.

FIG. 2 is an enlarged sectional view taken along the section line II-IIof FIG. 1. Its basic circuit structure is the same as that of FIG. 11described in connection with the conventional technology. The embodimentdiffers from the conventional technology in that the absolute value ofthe waveguide birefringence of the directional couplers 102 and 106constituting the first and second optical couplers is set at a valueequal to or greater than 3.5×10⁻⁴. Here, the waveguide birefringence Bequals the difference between the TM mode effective index n_(TM) and theTE mode effective index n_(TE) (B=n_(TM)−n_(TE)) as defined by theexpression (12).

The fabrication process of the waveguides of the embodiment will bedescribed briefly with reference to process diagrams of FIGS. 3A-3E.Under-cladding fine glass particles 301 mainly composed of SiO₂ and corefine glass particles 302 composed of SiO₂ doped with GeO₂ are depositedon a silicon substrate 109 by a flame hydrolysis deposition method (FHD)(see FIG. 3A). At this step, since the fine glass particles 301 and 302scatter light, they appear as an opacity film.

After that, the glass is made transparent at a high temperature above1,000° C. When the silicon substrate 109 having the fine glass particles301 and 302 deposited on its surface is heated gradually, the fine glassparticles melt, thereby forming a transparent glass film. In this case,the deposition amount of the fine glass particles are controlled in sucha manner that an under-cladding glass layer 303 becomes 30 μm thick, andthe core glass layer 304 becomes 7 μm thick (see FIG. 3B).

Continuously, the core glass layer 304 is patterned using thephotolithography technique and reactive ion etching (RIE). Thus, thecores 305 are formed on the under-cladding glass layer 303 (see FIG.3C).

Then, SiO₂ over-cladding fine glass particles 306 are deposited on theunder-cladding glass layer 303 and cores 305 by the flame hydrolysisdeposition method (FHD) (see FIG. 3D).

Finally, an embedded waveguide is formed which has a transparentover-cladding glass layer 307 made transparent through the hightemperature (see FIG. 3E). A dopant is added to the over-cladding glasslayer 307 to lower the glass transition temperature, thereby preventingthe cores 305 from changing the shape in the process of making theover-cladding glass layer 307 transparence under the high temperature.Here, the over-cladding glass layer (called “over-cladding layer” fromnow on) 307 and the under-cladding glass layer (called “under-claddinglayer” from now on) 303 shown in FIG. 3E correspond to the cladding 111of FIG. 2, and the cores 305 correspond to the waveguides (cores) 103and 104 of FIG. 2.

The waveguide birefringence depends on the aspect ratio of the cores305, the thermal expansion coefficients of the substrate 109, core glass305 and cladding glasses 303 and 307, and the softening temperature ofthe glasses. Accordingly, the waveguide birefringence can be controlledby appropriately selecting these values.

To fabricate the planar lightwave circuit type variable opticalattenuator, in addition to the steps described in connection with FIGS.3A-3E, the following steps are taken: a step of forming the thin filmheaters 108 as shown in FIG. 1 and FIG. 2 and wiring electrodes on thesurface of the over-cladding layer 307; and a step of forming thestress-releasing grooves 110 as shown in FIG. 1 and FIG. 2 to suppressthe polarization dependence due to the thermal stress generated by thethermooptic phase shifter 105.

Before describing a specific example of the embodiment in accordancewith the present invention, firstly, the polarization mode coupling inthe directional coupler sections is evaluated to demonstrate the PDLgeneration model described before. In addition, from the point of viewthat the polarization mode coupling may depend on the waveguidebirefringence, the correlation between them is obtained. FIG. 4illustrates the relationships between the polarization mode coupling andthe waveguide birefringence in the directional coupler sections. Here,the axis of abscissas represents the waveguide birefringence, and theaxis of ordinates represents the polarization mode coupling at the crossport output after passing through a single stage of the directionalcoupler. From FIG. 4, it is seen that the polarization mode coupling atthe cross port output has strong correlation with the waveguidebirefringence. The phenomenon can be interpreted in this way: “Becausethe mode coupling is inversely proportional to the propagation constantdifference (waveguide birefringence) between the two modes (twopolarization modes here) which bring about the coupling”. In addition,some variation in the polarization mode coupling against the samewaveguide birefringence in FIG. 4 can be interpreted in this way thatbecause the polarization mode coupling changes owing to a variety ofdisturbances.

Assuming that the polarization mode coupling in the optical couplers 102and 106, that is, |sin 2α|, be equal to or less than −25 dB, then theleft side value of the foregoing expression (16) becomes equal to orless than −25 dB against the input level at the maximum. In other words,the optical level depending on the polarization of the input light is−25 dB against the input level. As a result, the PDL at the opticalattenuation of 15 dB can be suppressed to less than or equal to the PDLobtained by the following expression (18). $\begin{matrix}{{PDL} = {{{ABS}\left\lbrack {10\quad{\log\left( \frac{10^{\frac{15}{10}} + {10^{\frac{25}{10}} \cdot \frac{1}{2}}}{10^{\frac{15}{10}} - {10^{\frac{25}{10}} \cdot \frac{1}{2}}} \right)}} \right\rbrack} = 0.43}} & (18)\end{matrix}$Thus, the PDL at the optical attenuation of 15 dB can be suppressed toless than or equal to 0.5 dB.

It is found from FIG. 4 that the polarization mode coupling can be madeless than or equal to −25 dB by setting the absolute value of thewaveguide birefringence at a value equal to or greater than 3.5×10⁻⁴.Thus, the PDL at the optical attenuation of 15 dB can be suppressed toless than or equal to 0.5 dB. Consequently, the present embodiment ischaracterized in that the polarization mode coupling of the first andsecond optical couplers 102 and 106 is made equal to or less than −25dB, and further fact that the waveguide birefringence (absolute value)of the directional couplers constituting the first and second opticalcouplers can be made equal to or greater than 3.5×10⁻⁴.

FIG. 5 illustrates the correlation between the attenuation and PDL ofthe planar lightwave circuit type variable optical attenuator actuallyfabricated as the first embodiment in accordance with the presentinvention. For comparison with the third embodiment which will bedescribed later, the present embodiment daringly sets the arm waveguidelength at 11 mm, about 2.5 times the optical beat length, which is mostdistant from the integer multiple condition of the optical beat length.

The birefringence of the actually fabricated waveguides was 3.5×10⁻⁴when measured by a Senarmont measuring system. In this case, as shown inFIG. 5, the PDL at 15 dB attenuation is 0.4 dB, which fulfills theinitial target value equal to or less than 0.5 dB.

The present embodiment is described by way of example of the device asshown in FIG. 1 and FIG. 2, which operates as the planar lightwavecircuit type variable optical attenuator. The device shown in FIG. 1 andFIG. 2, however, can be utilized as an optical switch by using the phasedifference between the two light beams entering the second opticalcoupler 106 at two values 0 and π. Likewise, the remaining embodimentsin accordance with the present invention, which will be described below,can also be utilized as an optical switch.

Second Embodiment

The second embodiment in accordance with the present invention has thesame basic construction as the first embodiment in accordance with thepresent invention as shown in FIG. 1 and FIG. 2: it is a variableoptical attenuator using its cross port output as the output waveguidefor the input waveguide. The first embodiment and second embodimentdiffer from each other in the following. The first embodiment ischaracterized by “setting the absolute value of the waveguidebirefringence of the directional couplers constituting the first andsecond optical couplers equal to or greater than 3.5×10⁻⁴”. In contrast,the second embodiment is characterized by “setting the length of the armwaveguides at an integer multiple of the optical beat length obtained bydividing the used optical wavelength by the waveguide birefringence”.

The second embodiment corresponds to the polarization dependentcondition given by the second expression of the foregoing expression(17).

The birefringence of the waveguide fabricated in the present embodimentwas 1.2×10⁻⁴. In this case, the optical beat length which causes thepolarization with the used optical wavelength of 1.55 μm to make onerevolution due to the waveguide birefringence is calculated as 12.9 mm.Thus, in the present embodiment, the length of the arm waveguide 104 isdesigned to be 12.9 mm corresponding to the optical beat length.

FIG. 6 illustrates the correlation between the attenuation and PDL ofthe planar lightwave circuit type variable optical attenuator actuallyfabricated by applying the foregoing design conditions in the presentembodiment. It is found from FIG. 6 that the PDL at 15 dB attenuationcan be suppressed to 0.9 dB, which is considerably smaller than that ofthe conventional example.

Third Embodiment

The third embodiment in accordance with the present invention is also avariable optical attenuator that employs the cross port output 107 b asthe output waveguide for the input waveguide 101 a as the first andsecond embodiments in accordance with the present invention. Its basicconstruction is the same as that of FIG. 1 and FIG. 2. The thirdembodiment is characterized by having both the characteristic of thefirst embodiment and that of the second embodiment. That is, it has thecharacteristic of “setting the waveguide birefringence (the absolutevalue) of the directional couplers constituting the first and secondoptical couplers equal to or greater than 3.5×10⁻⁴” and thecharacteristic of “setting the length of the arm waveguides at aninteger multiple of the optical beat length obtained by dividing theused optical wavelength by the waveguide birefringence”.

The birefringence of the actually fabricated waveguide was 3.5×10⁻⁴. Thelength of the arm waveguides was set at 13.3 mm, three times the opticalbeat length.

FIG. 7 illustrates the correlation between the attenuation and PDL ofthe planar lightwave circuit type variable optical attenuator actuallyfabricated in the third embodiment. It is found from FIG. 7 that the PDLat 15 dB attenuation can be suppressed to 0.2 dB and the PDL at 25 dBattenuation can be suppressed to 0.6 dB, which are very small values.Thus, “setting the waveguide birefringence (the absolute value) of thedirectional couplers constituting the first and second optical couplersequal to or greater than 3.5×10⁻⁴” and “setting the length of the armwaveguides at an integer multiple of the optical beat length obtained bydividing the used optical wavelength by the waveguide birefringence” canbe designed independently of each other. Accordingly, it is preferablethat the variable optical attenuator be fabricated in such a manner asto satisfy the two conditions at the same time.

Other Embodiments

The present invention has been described with respect to preferredembodiments. However, the embodiments of the present invention are notlimited to the foregoing examples, but include all the modificationssuch as the replacement, changes, additions, increase or decrease in thenumber, or design changes of shapes of the components as long as theyfall within the scope of the claims.

For example, although the foregoing embodiments in accordance with thepresent invention suppose the flame hydrolysis deposition method as afabrication method, this is not essential. For example, as fabricationmethods of the embedded waveguides other than the flame hydrolysisdeposition method, there are vapor phase growth methods such as a CVD(Chemical Vapor Deposition) method and VPE (Vapor Phase Epitaxy) method,and a physical deposition method such as a sputtering method. Thepresent invention is also effective when applying these fabricationmethods.

In addition, although the foregoing embodiments in accordance with thepresent invention suppose the directional couplers as the opticalcouplers, this is not essential. For example, multimode interferencecouplers or asymmetric X-type couplers are also effective as the opticalcouplers constituting the present invention because they cause thepolarization mode coupling in the regions where the cores are broughtinto close proximity to each other. In other words, the presentinvention is applicable to optical couplers that cause polarization modecoupling in the optical multi/demultiplexers independently of theirshapes.

Furthermore, although the foregoing embodiments in accordance with thepresent invention are described by way of example of the opticalinterferometer employing the silica-based glass waveguides formed on thesilicon substrate, this is not essential. For example, the principle ofthe present invention is also applicable to the devices where thewaveguide material is other than the glass such as polyimide, silicone,semiconductor and LiNbO₃. In addition, the material of the substrate isnot limited to silicon.

1. A planar lightwave circuit type variable optical attenuator havingwaveguides formed on a substrate, said variable optical attenuatorcomprising: an input waveguide; a first optical coupler; a secondoptical coupler; two arm waveguides connecting said first opticalcoupler to said second optical coupler; and an output waveguide, whereineach of said first optical coupler and said second optical coupler is adirectional coupler having a region in which said two arm waveguides arebrought in close proximity to each other; and a polarization modecoupling in said first optical coupler and said second optical coupleris equal to or less than −25 dB.
 2. The planar lightwave circuit typevariable optical attenuator as claimed in claim 1, wherein an absolutevalue of a waveguide birefringence at optical coupler sectionsconstituting said first optical coupler and said second optical coupleris equal to or greater than 3.5×10⁻⁴.
 3. The planar lightwave circuittype variable optical attenuator as claimed in claim 1, wherein a lengthof said arm waveguides is designed to be equal to an integer multiple ofan optical beat length obtained by dividing a used optical wavelength bythe waveguide birefringence.
 4. The planar lightwave circuit typevariable optical attenuator as claimed in claim 2, wherein a length ofsaid arm waveguides is designed to be equal to an integer multiple of anoptical beat length obtained by dividing a used optical wavelength bythe waveguide birefringence.
 5. The planar lightwave circuit typevariable optical attenuator as claimed in claim 1, wherein at least oneof said two arm waveguides has a phase controller; and said variableoptical attenuator functions as a variable optical attenuator or opticalswitch.
 6. The planar lightwave circuit type variable optical attenuatoras claimed in claim 2, wherein at least one of said two arm waveguideshas a phase controller; and said variable optical attenuator functionsas a variable optical attenuator or optical switch.
 7. The planarlightwave circuit type variable optical attenuator as claimed in claim3, wherein at least one of said two arm waveguides has a phasecontroller; and said variable optical attenuator functions as a variableoptical attenuator or optical switch.
 8. The planar lightwave circuittype variable optical attenuator as claimed in claim 4, wherein at leastone of said two arm waveguides has a phase controller; and said variableoptical attenuator functions as a variable optical attenuator or opticalswitch.
 9. The planar lightwave circuit type variable optical attenuatoras claimed in claim 1, wherein said substrate is a silicon substrate,and said waveguides are silica-based glass waveguides.
 10. The planarlightwave circuit type variable optical attenuator as claimed in claim2, wherein said substrate is a silicon substrate, and said waveguidesare silica-based glass waveguides.
 11. The planar lightwave circuit typevariable optical attenuator as claimed in claim 3, wherein saidsubstrate is a silicon substrate, and said waveguides are silica-basedglass waveguides.
 12. The planar lightwave circuit type variable opticalattenuator as claimed in claim 4, wherein said substrate is a siliconsubstrate, and said waveguides are silica-based glass waveguides.